There is known an obstacle detection system that causes a distance sensor to detect an obstacle approaching a vehicle to then warn an occupant of the vehicle of the obstacle approaching. For instance, as shown in FIG. 5, a vehicle is provided with distance sensors 101 to 104 at four corners of the vehicle, each of which has a detection zone including a short range, a middle range, and a long range. The vehicle is further provided with a first buzzer corresponding to the front-side sensors 101, 102 and a second buzzer corresponding to the rear-side sensors 103, 104. The first and second buzzers output sounds whose frequencies are different from each other, which enables determination whether an obstacle is present in a vicinity to the front corner of the vehicle or to the rear corner of the vehicle. For instance, as shown in FIG. 6, when a vehicle 100 is parallel parked, a left front sensor 101 and a left rear sensor 103 detect obstacles. In this case, the distinction between the two locations can be possible.
Under this obstacle detection system, when the number of buzzers is changed from two to one, the following problem occurs. FIG. 7 shows a combination of output patterns of a warning sound by the buzzer. The line shows each case where an obstacle is detected by the front-side sensors 101, 102 in a short range, in a middle range, or in a long range of the detection zone, or where no obstacle is detected. Similarly, the column shows each case where an obstacle is detected by the rear-side sensors 103, 104 in a short range, in a middle range, or in a long range of the detection zone, or where no obstacle is detected. Here, when both sensors do not detect an obstacle, the buzzer is being off so that no sound is outputted, as shown in (a) in FIG. 8. In contrast, when either sensor detects an obstacle, the buzzer is repeatedly turned on and off according to a distance. That is, the buzzer is repeatedly turned on and off with 0.15-second intervals in the long distance, as shown in (b) in FIG. 8; the buzzer is repeatedly turned on and off with 0.075-second intervals in the middle range, as shown in (c) in FIG. 8; and the buzzer is continuously turned on (or outputs a continuous sound) in the short range as shown in (d) in FIG. 8.
On the other hand, when both sensors detect obstacles, the buzzer is controlled as follows. In a case where one of the front-side sensor 101, 102 or the rear-side sensor 103, 104 detects an obstacle in the long range, the buzzer is repeatedly turned on and off with 0.15-second intervals when the other detects an obstacle in the long range, as shown in (b) in FIG. 8; the buzzer is repeatedly turned on and off with 0.075-second intervals when the other detects an obstacle in the middle range, as shown in (c) in FIG. 8; and the buzzer is continuously turned on (or outputs a continuous sound) when the other detects an obstacle in the short range, as shown in (d) in FIG. 8. Further, In a case where one of the front-side sensor 101, 102 or the rear-side sensor 103, 104 detects an obstacle in the middle range, the buzzer is repeatedly turned on and off with 0.075-second intervals when the other detects an obstacle in the middle range, as shown in (c) in FIG. 8; and the buzzer is continuously turned on (or outputs a continuous sound) when the other detects an obstacle in the short range as shown in (d) in FIG. 8. Furthermore, in a case where one of the front-side sensor 101, 102 or the rear-side sensor 103, 104 detects an obstacle in the short range, the buzzer is continuously turned on (or outputs a continuous sound) when the other detects an obstacle in the short range, as shown in (d) in FIG. 8.
These warning patterns cannot detect that obstacles are present in both of the front side and the rear side, so that the vehicle has possibility of contacting whichever of the obstacles is closer to the vehicle. To avoid this, if all different warning patterns are prepared for all the combination when the both of the front-side sensor 101, 102 and the rear-side sensor 103, 104 detect obstacles, nine patterns must be prepared. However, it is not realistically favorable because, for an occupant of the vehicle, it is very burdensome to judge the warning patterns of no fewer than nine and thereby very difficult to properly determine the circumstance.